Réponse :
Bonjour,
a) Joining the army meant a lot to him.
b) Being from an hispanic background she strived to fit into society.
c) Working with people from different backgrounds makes him happy.
d) After resolving their diversity problem , they became the best on the market.
Explications :
to join the army = s'engager dans l'armée
to enlist the army = s'engager dans l'armée
; )